Communication network and design method

ABSTRACT

A communication network includes a starting node that has a variable dispersion compensator that performs dispersion compensation at a variable dispersion compensation amount such that a residual dispersion amount of an optical signal transmitted therethrough becomes a predetermined reference residual dispersion amount; and plural nodes that are subjected to dispersion compensation design using the starting node as a starting point and that include fixed dispersion compensators selected based on the reference residual dispersion amount.

FIELD

The embodiments discussed herein are related to a communication networkand a design method.

BACKGROUND

Wavelength division multiplexing (WDM) transmitting apparatuses areincreasingly in demand with recent increases in traffic in communicationnetworks. WDM transmitting apparatuses have been actively introduced inlocal networks (metro networks).

Although a local network typically takes a form of a ring communicationnetwork, it is projected that a shift to mesh communication networkswill be made to flexibly support traffic demands in the future. If anoptical signal of 10 Gb/s or more is propagated over a long distance,the optical waveform deteriorates due to nonlinear optical effects suchas wavelength dispersion in the optical fiber and self phase modulation(SPM) generated in the optical fiber.

To compensate the deterioration of the optical waveform due towavelength dispersion, dispersion compensation by a dispersioncompensator is performed. Dispersion compensators utilized include, forexample, a dispersion compensating fiber (DCF) having a fixed dispersioncompensation amount and a virtually imaged phased array (VIPA) variabledispersion compensator having a variable dispersion compensation amount.

In ring and mesh communication networks, optical add drop multiplexers(OADM) are used for inserting an optical signal transmitted from anothercommunication network and branching an optical signal to anothercommunication network (see, for example, Japanese Laid-Open PatentPublication No. 2006-135788 and International Publication Pamphlet No.2004/098102).

FIG. 14 is a block diagram of a functional configuration of aconventional ring communication network. As depicted in FIG. 14, aconventional communication network 1400 is reconfigurable OADM (ROADM)made up of four nodes #1 to #4 connected in a ring shape. ROADM is oneform of OADM and is a remote-wavelength-controllable wavelengthmultiplexing device.

The communication network 1400 transmits a WDM optical signal, which isa wavelength-multiplexed optical signal, and branches an optical signalof each wavelength (channel) included in the WDM optical signal toanother communication network or inserts an optical signal transmittedfrom another communication network into the WDM optical signal. Each ofthe nodes #1 to #4 is a ROADM node including a fixed dispersioncompensator.

A path 1410 is a path of an optical signal inserted from anothercommunication network into the node #1, passing through the nodes #2 to#4, and returning to the node #1. A path 1420 indicates a path of anoptical signal inserted from another communication network into the node#4, passing through the nodes #1 to #3, and branched from the node #3 toanother communication network.

FIG. 15 is a block diagram of a functional configuration of a ROADMnode. A ROADM node 1500 is an exemplary configuration of each of thenodes #1 to #4 depicted in FIG. 14 and branches (drops) a portion of theWDM optical signal wavelength-multiplexed and transmitted from anotherROADM node of the communication network 1400 and transmits the portionto another communication network through an interface unit 1560.

The ROADM node 1500 receives an optical signal transmitted from anothercommunication network through the interface unit 1560 and inserts (adds)the optical signal into the WDM optical signal passing through the ROADMnode 1500. A fixed dispersion compensator 1501 performs dispersioncompensation, by a fixed dispersion compensation amount, on a WDMoptical signal transmitted from another ROADM node of the communicationnetwork 1400.

FIG. 16 is a diagram depicting changes in a cumulative residualdispersion amount in a communication network. In FIG. 16, the horizontalaxis indicates the distance of a path of an optical signal and the nodes#1 to #4 through which the optical signal passes. The vertical axisindicates the cumulative residual dispersion amount of the opticalsignal passing through the paths 1410, 1420 depicted in FIG. 14.

A dotted line 1610 a indicates changes in the cumulative dispersionamount when the optical signal is inserted from the node #1 as in thecase of the path 1410. The nodes #1 to #4 are equipped with respectivefixed dispersion compensators and the cumulative dispersion amount isreduced at each of the nodes #1 to #4. As a result, the cumulativedispersion amount changes at the nodes #1 to #4 as indicated by thedotted line 1610 a.

A dotted line 1620 a indicates changes in the cumulative dispersionamount when an optical signal is inserted from the node #4 as in thecase of the path 1420. As a result, the cumulative dispersion amountchanges in the nodes #1 to #4 as indicated by the dotted line 1620 a.Reference numeral 1630 denotes an ideal residual dispersion amount(hereinafter, “RDtgt”) in the nodes #1 to #4 when the optical signal isinserted from the node #1.

Reference numeral 1640 denotes dispersion tolerance in the communicationnetwork 1400 when the optical signal is inserted from the node #1. Thedispersion tolerance is a range of the residual dispersion amountnecessary for acquiring predetermined characteristics on the receivingside. Reference numeral 1650 denotes RDtgt in the communication network1400 when the optical signal is inserted from the node #4.

Due to the nonlinear optical effects such as self-phase modulationgenerated in the optical fiber, chirp is generated. Therefore, asdenoted by reference numerals 1630, 1640, and 1650, the RDtgt and thedispersion tolerance in the communication network vary depending on thenumber of the nodes through which the optical signal passes and the spanbetween the nodes.

FIG. 17 is a diagram depicting dispersion compensation design in aconventional ring communication network. In FIG. 17, the horizontal axisindicates nodes #1 to #4 through which the optical signal passes. Thevertical axis indicates a deviation amount between the residualdispersion amount and RDtgt of the optical signal passing through thepaths 1410, 1420. It will hereinafter be assumed that the residualdispersion of the optical signal inserted into the communication network1400 is RDtgt.

A solid line 1710 is a design example of the dispersion compensationusing the node #1 as a starting node. On the assumption that an opticalsignal having the residual dispersion amount of RDtgt is inserted fromanother communication network to the node #1, the fixed dispersioncompensators are selected in the order of the node #2, the node #3, thenode #4, and the node #1.

A solid line 1720 indicates the deviation amount between the residualdispersion amount and RDtgt of the optical signal inserted from anothercommunication network to the node #4 and branched from the node #3.

FIG. 18 is a block diagram of a functional configuration of aconventional mesh communication network. As depicted in FIG. 18, aconventional communication network 1800 is a mesh communication networkconnecting a ring #1 and a ring #2. The ring #1 and the ring #2 eachhave the same configuration as the conventional communication network1400 depicted in FIG. 14.

The ring #1 is made up of nodes #11 to #15, each of which includes afixed dispersion compensator. The ring #2 is made up of nodes #21 to#25, each of which includes a fixed dispersion compensator. The node #12of the ring #1 and the node #24 of the ring #2 are connected to eachother and are hub nodes connecting the ring #1 and the ring #2.

A path 1810 is a path of an optical signal inserted from anothercommunication network into the node #11 of the ring #1, passing throughthe nodes #12 to #15, and returning to the node #11. A path 1820 is apath of an optical signal inserted from another communication networkinto the node #21 of the ring #2, passing through the nodes #22 to #25,and returning to the node #21.

A path 1830 is a path of an optical signal inserted from anothercommunication network into the node #15 of the ring #1, passing throughthe node #11 and the node #12, branched to the ring #2, passing throughthe node #24, the node #25, and the node #21 of the ring #2, andbranched from the node #21 to another communication network.

FIG. 19 is a block diagram of a functional configuration of the hubnodes. In FIG. 19, constituent elements identical to those depicted inFIG. 15 are given the same reference numerals used in FIG. 15 and willnot be described. The node #12 of the ring #1 and the node #24 of thering #2 each have a configuration identical to that of the ROADM node1500 depicted in FIG. 15 and includes a fixed dispersion compensator1501.

FIG. 20 is a diagram depicting dispersion compensation design in aconventional mesh communication network. In FIG. 20, reference numeral2001 denotes characteristics of a deviation amount between the residualdispersion amount and RDtgt of the optical signal in the ring #1.Reference numeral 2002 denotes characteristics of a deviation amountbetween the residual dispersion amount and RDtgt of the optical signalin the ring #2.

A solid line 2010 indicates a design example of the dispersioncompensation using the node #11 as a starting node. A solid line 2020indicates a design example of the dispersion compensation using the node#21 as a starting node.

A heavy line 2030 indicates the deviation amount between the residualdispersion amount and RDtgt of the optical signal inserted from the node#15 of the ring #1, passing through the node #11 and the node #12,branched to the ring #2 (reference numeral 2003), passing through thenode #24, the node #25, and the node #21 of the ring #2, and branchedfrom the node #21 as in the case of the path 1830.

However, it is problematic in the above conventional technology that theamount of deviation between the residual dispersion amount and RDtgtincreases depending on the amount of dispersion compensation by a fixeddispersion compensator. For example, in the ring communication network1400 depicted in FIG. 14, Δ is assumed as a step amount of thedispersion compensation amount of the fixed dispersion compensator 1501included in the nodes #1 to #4. In this case, as depicted in FIG. 17,the deviation amount between the residual dispersion amount and RDtgt ofthe optical signal is ±Δ/2 at maximum in the nodes #1 to #4 for theoptical signal inserted from the node #1.

Sine the dispersion compensation design is performed assuming that theresidual dispersion amount of the optical signal passing through thenode #1 is RDtgt, if the residual dispersion amount of the opticalsignal passing through the node #1 is not RDtgt, it is problematic thatthe deviation amount between the residual dispersion amount and RDtgt isincreased according to the fixed amount of the dispersion compensationby the fixed dispersion compensator 1501 included in the node #1.

For example, in the communication network 1400 designed as in the designexample 1710 of FIG. 17, it is assumed that an optical signal having theresidual dispersion amount of RDtgt is inserted from anothercommunication network to the node #4. In this case, the residualdispersion amount of the optical signal passing through the node #1 isnot RDtgt depending on the step amount Δ of the fixed dispersioncompensator 1501 of the node #1.

Therefore, a residual dispersion amount in the nodes #1 to #4 isgenerated as indicated by reference numeral 1720 and the deviationamount between the residual dispersion amount and RDtgt becomes ±3Δ/2 atmaximum in the nodes #1 to #4. Therefore, if the step amount Δ of thefixed dispersion compensator is increased, the deviation amount betweenthe residual dispersion amount and RDtgt increases in the branchedoptical signal.

In the mesh communication network 1800 depicted in FIG. 18, if theresidual dispersion amount of the optical signal passing through the hubnode is not RDtgt, it is problematic that the deviation amount betweenthe residual dispersion amount and RDtgt is increased. For example, inthe communication network 1800 designed as in the design examples 2010and the design example 2020 of FIG. 20, it is assumed that an opticalsignal is transmitted through a path over the ring #1 and the ring #2.

In this case, the residual dispersion amount of the optical signalpassing through the node #12 is not RDtgt according to the step amount Δof the fixed dispersion compensator of the node #12. Therefore, aresidual dispersion amount in the nodes is generated as indicated by theheavy line 2030 and the deviation amount between the residual dispersionamount and RDtgt becomes ±5Δ/2 at maximum in the nodes. Therefore, ifthe step amount Δ of the fixed dispersion compensator is increased, thedeviation amount between the residual dispersion amount and RDtgtincreases in the branched optical signal.

Therefore, it is problematic that communication characteristicsdeteriorate since the deterioration of the optical signal increases dueto the wavelength dispersion. If the step amount Δ of the fixeddispersion compensator is reduced to diminish the deviation between theresidual dispersion amount and RDtgt of the branched optical signal, itis problematic that the costs of design and maintenance of thecommunication networks increase since the number and the types ofnecessary fixed dispersion compensators increase.

Although it is conceivable that a variable dispersion compensator isused for diminishing the deviation amount between the residualdispersion amount and RDtgt of the branched optical signal, the cost ofthe communication networks increases if variable dispersion compensatorsare applied to all the nodes since variable dispersion compensators aregenerally expensive. If variable dispersion compensators are used, sincethe eye opening deteriorates in the optical signal passing through amultiplicity of the dispersion compensators due to the passing bandcharacteristics thereof, arising in a problem in that the communicationcharacteristics deteriorate.

SUMMARY

According to an aspect of an embodiment, a communication networkincludes a starting node that has a variable dispersion compensator thatperforms dispersion compensation at a variable dispersion compensationamount such that a residual dispersion amount of an optical signaltransmitted therethrough becomes a predetermined reference residualdispersion amount; and plural nodes that are subjected to dispersioncompensation design using the starting node as a starting point and thatinclude fixed dispersion compensators selected based on the referenceresidual dispersion amount.

The object and advantages of the invention will be realized and attainedby means of the elements and combinations particularly pointed out inthe claims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and arenot restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram of a functional configuration of acommunication network according to a first embodiment;

FIG. 2 is a block diagram of a functional configuration of a ROADM node;

FIG. 3 is a flowchart of dispersion compensation design of thecommunication network according to the first embodiment;

FIG. 4 is diagram depicting the dispersion compensation design (thenumber of nodes is k) of the communication network according to thefirst embodiment;

FIG. 5 depicts the dispersion compensation design (the number of nodesis four) of the communication network according to the first embodiment;

FIG. 6 is a block diagram of a functional configuration of acommunication network according to an example of the first embodiment;

FIG. 7 is a table of exemplary design values of the communicationnetwork depicted in FIG. 6;

FIG. 8 is a block diagram of a functional configuration of acommunication network according to a second embodiment;

FIG. 9 is a block diagram of a functional configuration of hub nodes;

FIG. 10 is diagram depicting dispersion compensation design in thecommunication network according to the second embodiment;

FIG. 11 is a block diagram of a functional configuration of acommunication network according to an example of the second embodiment;

FIG. 12 is a table of exemplary design values of ring #1 depicted inFIG. 6;

FIG. 13 a table of exemplary design values of ring #2 depicted in FIG.11;

FIG. 14 is a block diagram of a functional configuration of aconventional ring communication network;

FIG. 15 is a block diagram of a functional configuration of a ROADMnode;

FIG. 16 is a diagram depicting changes in a cumulative residualdispersion amount in a communication network;

FIG. 17 is a diagram depicting dispersion compensation design in aconventional ring communication network;

FIG. 18 is a block diagram of a functional configuration of aconventional mesh communication network;

FIG. 19 is a block diagram of a functional configuration of hub nodes;and

FIG. 20 is a diagram depicting dispersion compensation design in aconventional mesh communication network.

DESCRIPTION OF EMBODIMENTS

Preferred embodiments of the present invention will be explained withreference to the accompanying drawings.

FIG. 1 is a block diagram of a functional configuration of acommunication network according to a first embodiment. As depicted inFIG. 1, a communication network 100 according to the first embodiment isROADM made up of k nodes #1 to #k connected in a ring shape. Thecommunication network 100 is a communication network subject todispersion compensation design using the node #1 as a starting node.

The communication network 100 transmits a WDM optical signal, which is awavelength-multiplexed optical signal, and branches each wavelength(channel) of the WDM optical signal to another communication network orinserts an optical signal transmitted from another communicationnetwork. The node #1 is a ROADM node that includes a variable dispersioncompensator. Each of the nodes #2 to #k is a ROADM node that includes afixed dispersion compensator (see FIG. 15).

A path 110 is a path of an optical signal inserted from anothercommunication network into the node #1, passing through the node #2, thenode #3, the node #4, . . . , and the node #k, and returning to the node#1. A path 120 indicates a path of an optical signal inserted fromanother communication network into the node #4, passing through the node#k, the node #1, the node #2, and the node #3, and branched from thenode #3 to another communication network.

FIG. 2 is a block diagram of a functional configuration of a ROADM node.A ROADM node 200 is an exemplary configuration of the node #1 depictedin FIG. 1 and includes a preamplifying unit 210, a wavelengthdemultiplexer 220, a add/drop unit 230 (Add/Drop), a wavelengthmultiplexer 240, and a post-amplifying unit 250. The add/drop unit 230is connected to an interface unit 260 that performs transmission withanother communication network.

The ROADM node 200 branches (drops) a portion of the WDM optical signalwavelength-multiplexed and transmitted from another ROADM node (the node#k) of the communication network 100 and transmits the portion toanother communication network through the interface unit 260. The ROADMnode 200 receives an optical signal transmitted from anothercommunication network through the interface unit 260 and inserts (adds)the optical signal into the WDM optical signal passing through the ROADMnode 200.

The preamplifying unit 210 includes a variable dispersion compensator211 and an amplifier 212. The variable dispersion compensator 211performs dispersion compensation by a variable dispersion compensationamount with respect to a WDM optical signal transmitted from anothernode of the communication network 100. The variable dispersioncompensator 211 is a Fiber Bragg Gating (FBG), a VIPA plate, or a ringresonator, for example.

The variable dispersion compensator 211 outputs thedispersion-compensated optical signal to the amplifier 212. Theamplifier 212 amplifies and outputs to the wavelength demultiplexer 220,the optical signal output from the variable dispersion compensator 211.The wavelength demultiplexer 220 demultiplexes the optical signal outputfrom the preamplifying unit 210. The wavelength demultiplexer 220outputs each of the demultiplexed optical signals to the add/drop unit230.

The add/drop unit 230 individually outputs each of the optical signalsoutput from the wavelength demultiplexer 220 through switching of aswitch not depicted, etc., to the wavelength multiplexer 240 or theinterface unit 260. The add/drop unit 230 outputs the optical signaloutput from the interface unit 260 to the wavelength multiplexer 240.

The wavelength multiplexer 240 wavelength-multiplexes each of theoptical signals output from the add/drop unit 230. The wavelengthmultiplexer 240 outputs the wavelength-multiplexed WDM optical signal tothe post-amplifying unit 250. The post-amplifying unit 250 amplifies andtransmits the WDM optical signal output from the wavelength multiplexer240 to another node (the node #2) of the communication network 100.

The interface unit 260 is made up of transponders. The interface unit260 transmits the optical signal output from the add/drop unit 230 toanother communication network through the transponders. The interfaceunit 260 outputs to the add/drop unit 230, the optical signal receivedfrom another communication network, through a transponder.

The dispersion compensation design of the communication network 100 willbe described. It will hereinafter be assumed that the residualdispersion of the optical signal inserted into the communication network100 is RDtgt (reference residual dispersion amount). In the dispersioncompensation design of the communication network 100, the dispersioncompensation amounts of the nodes #1 to #k are designed such that thedispersion compensation amounts of the optical signals branched from therespective nodes #1 to #k come closer to RDtgt.

The node #1 having the variable dispersion compensator 211 is determinedas a starting node for designing the dispersion compensation amount.Fixed dispersion compensators of the nodes #2 to #k are selected suchthat a deviation amount between the residual dispersion amount and RDtgtof the branched optical signal is minimized regardless of which node theoptical signal inserted into the node #1 is branched from among thenodes #2 to #k. This allows the residual dispersion amount of thebranched signal to fall within a dispersion tolerance (predeterminedrange).

It is assumed that RD(n) denotes a residual dispersion amount of anoptical signal inserted from the node #1 and branched from the node #n.It is assumed that RDtgt(n) denotes the optimum residual dispersionamount of an optical signal branched from a node #n. A deviation amountd(1,n) between the residual dispersion amount and RDtgt of the opticalsignal inserted from the node #1 and branched from the node #n may berepresented by equation (1).

d(1,n)=RD(n)−RDtgt(n)  (1)

Assuming that k denotes the number of nodes making up the communicationnetwork 100, a deviation amount d(i,j) between the residual dispersionamount and RDtgt of an optical signal inserted from a node #i andbranched from a node #j may be represented by equation (2).

d(i,j)=−d(1,i)+d(1,j)+d(1,k+1)  (2)

where i, j=1, 2, . . . , k.

A first term of the right-hand side of equation (2) indicates thedeviation amount between the residual dispersion amount and RDtgt of theoptical signal inserted from the node #1 and branched from the node #i.A second term indicates the deviation amount between the residualdispersion amount and RDtgt of the optical signal inserted from the node#1 and branched from the node #j.

A third term indicates the deviation amount between the residualdispersion amount and RDtgt of the optical signal inserted from the node#1 and branched from the node #1. The deviation amount of the third termis a deviation amount between a residual dispersion amount and RDtgtwhen an optical signal inserted into the node #1 passes through the node#2, . . . , the node #k, and the node #1 to be branched from the node#1, for example.

FIG. 3 is a flowchart of the dispersion compensation design of thecommunication network according to the first embodiment. As depicted inFIG. 3, the node #1 is determined as a starting node that is thestarting point of the dispersion compensation design (step S301). RDtgt(see FIG. 16) in each of the nodes #1 to #k is calculated (step S302).RDtgt (predetermined reference residual dispersion amount) in each ofthe nodes #1 to #k is preliminarily calculated based on information of atransmission path or acquired from a database.

The node #n subjected to the dispersion compensation design is changedto the node #2 (n=2) (step S303). Information is acquired for awavelength dispersion amount generated in a transmission path betweenthe node #n−1 and the node #n (step S304). The information of thewavelength dispersion amount generated in the transmission path betweenthe node #n−1 and the node #n is calculated based on information of thespan and characteristics of the transmission path between the node #n−1and the node #n.

An ideal dispersion compensation amount for the node #n is thencalculated (step S305). The ideal dispersion compensation amount of thenode #n is a dispersion compensation amount when the deviation amountd(1,n) is zero between the residual dispersion amount and RDtgt of theoptical signal inserted from the node #1 and branched from the node #n.The ideal dispersion compensation amount of the node #n is calculatedbased on the residual dispersion amount of the optical signal branchedfrom the node #n−1 and the information of the wavelength dispersionamount calculated at step S304.

It is determined whether the node #n under the dispersion compensationdesign is the node #1 (n=k+1) (step S306). If the node #n is not thenode #1 (step S306: NO), the fixed dispersion compensator of the node #nis selected such that d(1,n) is minimized (step S307). The node #nsubjected to the dispersion compensation design is changed to the node#n+1 (n=n+1) (step S308) and the process returns to step S304 andcontinues.

If the node #n is the node #1 at step S306 (step S306: YES), thedispersion compensation amount of the variable dispersion compensator ofthe node #1 is set such that the deviation amount d(1,k+1) becomes zerobetween the residual dispersion amount and RDtgt of the optical signalinserted from the node #1, passing through the nodes #2 to #k, andbranched from the node #1 (step S309), and the dispersion compensationdesign of the communication network is terminated.

FIG. 4 depicts the dispersion compensation design (the number of nodesis k) of the communication network according to the first embodiment.FIG. 4 depicts the dispersion compensation design when the number ofnodes of the communication network 100 is k (see FIG. 1). In FIG. 4, thehorizontal axis indicates the nodes #1 to #k through which the opticalsignal passes. The vertical axis indicates a deviation amount betweenthe residual dispersion amount and RDtgt of the optical signal.

A solid line 410 indicates the dispersion compensation design using thenode #1 as the starting node. On the assumption that the optical signalhaving the residual dispersion amount of RDtgt is inserted into the node#1, fixed dispersion compensators are selected in the order of the node#2, the node #3, . . . , and the node #k such that the deviation amountsd(1,2) to d(1,k) between the residual dispersion amounts and RDtgt ofthe optical signals branched from the nodes are minimized.

If a fixed dispersion compensator having a step amount of Δ is used,since RD(n) of the right-hand side of the equation (1) may be adjustedby Δ, the maximum value of d(1,n) of the left-hand side of the equation(1) may be constrained to ±Δ/2 by selecting the optimum fixed dispersioncompensator. Therefore, the maximum value of the deviation amount d(1,n)may be constrained to ±Δ/2 between the residual dispersion amount andRDtgt of the optical signal inserted from the node #1 and branched fromany one of the nodes #2 to #k.

Each of the first, second, and third terms of the right-hand side of theequation (2) is ±Δ/2 at maximum. As denoted by reference numeral 411,the third term of the right-hand side of the equation (2) may be set tozero by setting the dispersion compensation amount of the variabledispersion compensator 211 of the node #1 such that d(1,k+1) becomeszero. Therefore, the maximum value of the deviation amount d(i,j) may beconstrained to ±Δ between the residual dispersion amount and RDtgt ofthe optical signal inserted from the node #i and branched from any oneof the nodes #j.

For example, a heavy line 420 indicates the deviation amount between theresidual dispersion amount and RDtgt of the optical signal inserted fromthe node #4, passing through the node #k and the nodes #1 to #3, andbranched from the node #3 as in the case of the path 120 of FIG. 1. Asindicated by the heavy line 420, if the optical signal is inserted fromthe node #4, d(4,3) having the largest d(i,j) may be constrained to −Δalthough the residual dispersion amount of the optical signal passingthrough the node #1 is not RDtgt.

FIG. 5 depicts the dispersion compensation design (the number of nodesis four) of the communication network according to the first embodiment.FIG. 5 depicts the dispersion compensation design when the number ofnodes of the communication network 100 is four (see FIG. 14). In FIG. 5,the portions identical to those depicted in FIG. 4 are given the samereference numerals used in FIG. 4 and will not be described. A dottedline 510 indicates the dispersion compensation design using the node #1as the starting node (see FIG. 17) on the assumption that the dispersioncompensator included in the node #1 is the fixed dispersion compensator.

A heavy dotted line 520 indicates the deviation amount between theresidual dispersion amount and RDtgt of the optical signal inserted fromthe node #4, passing through the nodes #1 to #3, and branched from thenode #3 when it is assumed that the dispersion compensator included inthe node #1 is the fixed dispersion compensator. As depicted by theheavy dotted line 520, when it is assumed that the dispersioncompensator included in the node #1 is the fixed dispersion compensator,the deviation amount is −3Δ/2 between the residual dispersion amount andRDtgt of the optical signal inserted from the node #4, passing throughthe nodes #1 to #3, and branched from the node #3.

On the other hand, as depicted by the heavy line 420, if the dispersioncompensator included in the node #1 is the variable dispersioncompensator 211, the deviation amount is −Δ between the residualdispersion amount and RDtgt of the optical signal inserted from the node#4, passing through the nodes #1 to #3, and branched from the node #3.Therefore, as indicated by reference numeral 521, the deviation amountbetween the dispersion amount and RDtgt is improved by 33% when the node#1 is equipped with the variable dispersion compensator 211.

FIG. 6 is a block diagram of a functional configuration of acommunication network according to an example of the first embodiment.As depicted in FIG. 6, it is assumed that the number of nodes of thecommunication network 100 according to the example of the firstembodiment is four and that the nodes are nodes N11 to N14. It isassumed that a transmission path between the node N11 and the node N12is a transmission path S11, that a transmission path between the nodeN12 and the node N13 is a transmission path S12, that a transmissionpath between the node N13 and the node N14 is a transmission path S13,and that a transmission path between the node N14 and the node N11 is atransmission path S14.

The node N11 is the starting node of the dispersion compensation designof the communication network 100 and includes the variable dispersioncompensator 211. The nodes N12 to N14 are nodes subjected to thedispersion compensation design using the node N11 as the starting pointand include the fixed dispersion compensators. The transmission pathsS11 to S14 are assumed to be single mode fibers (SMF) having awavelength dispersion coefficient of 17 ps/nm/km. The number of steps ofthe fixed dispersion compensators included in the nodes N12 to N14 isassumed to be 200 ps/nm.

FIG. 7 depicts exemplary design values of the communication networkdepicted in FIG. 6. In FIG. 7, an item 710 indicates the spans of thetransmission paths S11 to S14. An item 720 indicates wavelengthdispersion amounts generated in the transmission paths S11 to S14. Anitem 730 indicates ideal dispersion compensation amounts of the nodesN11 to N14. An item 740 indicates dispersion compensation amounts of thenodes N11 to N14. An item 750 indicates deviation amounts d(i,j) betweenthe residual dispersion amount and RDtgt of the optical signal branchedfrom the nodes N11 to N14.

The wavelength dispersion amounts 720 generated in the transmissionpaths S11 to S14 are calculated. From the multiplication of the spans710 of the transmission paths S11 to S14 and the wavelength dispersioncoefficient of 17 ps/nm/km, the wavelength dispersion amounts 720generated in the transmission paths S11 to S14 may be calculated asfollows:

S11:561 ps/nm

S12:935 ps/nm

S13:1122 ps/nm

S14:1496 ps/nm

The fixed dispersion compensators of the nodes N12 to N14 are selected.The ideal dispersion compensation amounts 730, the actual dispersioncompensation amounts 740, and the deviation amounts 750 from RDtgt ofthe nodes N12 to N14 may be calculated as follows:

N12: the ideal dispersion compensation amount 561 ps/nm; the actualcompensation amount 600 ps/nm; from RDtgt, the deviation amountd(N11,N12)−39 ps/nm

N13: the ideal dispersion compensation amount 896 ps/nm; the actualcompensation amount 800 ps/nm; from RDtgt, the deviation amountd(N11,N13)96 ps/nm

N14: the ideal dispersion compensation amount 1218 ps/nm; the actualcompensation amount 1200 ps/nm; from RDtgt, the deviation amountd(N11,N14)18 ps/nm

The dispersion compensation amount is set for the variable dispersioncompensator 211 of the node N11, which is the starting node. The idealdispersion compensation amount 730, the actual dispersion compensationamount 740, and the deviation amount 750 from RDtgt of the node N11 maybe calculated as follows:

N11: the ideal dispersion compensation amount 1514 ps/nm; the actualcompensation amount 1514 ps/nm; from RDtgt, the deviation amount 0ps/nm.

From the above design, the deviation amount d(Ni,Nj) between theresidual dispersion amount and RDtgt of the optical signal inserted fromthe node Ni and branched from the node Nj may be calculated as followsand d(Ni,Nj) consistently falls within ±200 ps/nm:

the deviation amount d(N12,N13) in the path ofN12→N13=−d(N11,N12)+d(N11,N13)=39+96=135 ps/nm

the deviation amount d(N12,N14) in the path ofN12→N14=−d(N11,N12)+d(N11,N14)=39+18=57 ps/nm

the deviation amount d(N12,N11) in the path ofN12→N11=−d(N11,N12)+d(N11,N11)=39+0=39 ps/nm

the deviation amount d(N13,N14) in the path ofN13→N14=−d(N11,N13)+d(N11,N14)=−96+18=−78 ps/nm

the deviation amount d(N13,N11) in the path ofN13→N11=−d(N11,N13)+d(N11,N11)=−96+0=−96 ps/nm

the deviation amount d(N13,N12) in the path ofN13→N12=−d(N11,N13)+d(N11,N12)=−96−36=−135 ps/nm

the deviation amount d(N14,N11) in the path ofN14→N11=−d(N11,N14)+d(N11,N11)=−18+0=−18 ps/nm

the deviation amount d(N14,N12) in the path ofN14→N12=−d(N11,N14)+d(N11,N12)=−18−39=−57 ps/nm

the deviation amount d(N14,N13) in the path ofN14→N13=−d(N11,N14)+d(N11,N13)=−18+96=78 ps/nm

According to the communication network of the first embodiment, sincethe starting node of the dispersion compensation design of thecommunication network has the variable dispersion compensator, themaximum deviation amount between the residual dispersion amount andRDtgt may be constrained to the step amount Δ of the fixed dispersioncompensator in the transmissions among all the nodes of thecommunication network. Therefore, the communication characteristics maybe improved by constraining the deterioration of optical signals due tothe wavelength dispersion.

Since the variable dispersion compensator is applied to the startingnode alone among the nodes of the communication network, the cost of thecommunication network is reduced. For example, if VIPA is used as thevariable dispersion compensator, VIPA is applied to the starting nodealone among the nodes of the communication network, the eye opening ofthe optical signal does not deteriorate and the communicationcharacteristics is improved.

FIG. 8 is a block diagram of a functional configuration of acommunication network according to a second embodiment. As depicted inFIG. 8, a communication network 800 according to the second embodimentis a mesh communication network connecting a ring #1 and a ring #2. Thering #1 and the ring #2 each have a configuration identical to that ofthe communication network 100 according to the first embodiment.

The ring #1 and the ring #2 are each made up of nodes #1 to #k. Nodes #Hincluded in both the ring #1 and the ring #2 are connected to each otherand are hub nodes connecting the ring #1 and the ring #2. The nodes #Hare ROADM nodes that include a variable dispersion compensator.

The respective nodes #1 of the ring #1 and the ring #2 are the startingnodes of the dispersion compensation design of the ring #1 and the ring#2, respectively and are ROADM nodes that include a variable dispersioncompensator. The nodes #2, #k−1, and #k are ROADM nodes (see FIG. 15)that include fixed dispersion compensators.

A path 810 is a path of an optical signal inserted from anothercommunication network into the node #1 of the ring #1, passing throughthe node #2, . . . , the node #H, and the node #k, and returning to thenode #1. A path 820 is a path of an optical signal inserted from anothercommunication network into the node #1 of the ring #2, passing throughthe node #2, the node #3, the node #H, . . . , and the node #k, andreturning to the node #1.

A path 830 is a path of an optical signal inserted from anothercommunication network into the node #1 of the ring #1, passing throughthe node #2 and the node #H, branched to the ring #2, passing throughthe node #H, the node #k, and the node #1 of the ring #2, and branchedfrom the node #1 to another communication network.

FIG. 9 is a block diagram of a functional configuration of the hubnodes. In FIG. 8, constituent elements identical to those depicted inFIG. 2 are given the same reference numerals used in FIG. 2 and will notbe described. The respective nodes #H of the ring #1 and the ring #2have a configuration identical to that of the ROADM node 200 depicted inFIG. 2 and include the fixed dispersion compensator 211.

The add/drop unit 230 of the node H of the ring #1 outputs therespective optical signals output from the wavelength demultiplexer 220to the wavelength multiplexer 240 or the ring #2. The add/drop unit 230of the node H of the ring #1 outputs the optical signal output from thering #2 to the wavelength multiplexer 240 of the node #H of the ring #1.

The add/drop unit 230 of the node H of the ring #2 outputs therespective optical signals output from the wavelength demultiplexer 220to the wavelength multiplexer 240 or the ring #1. The add/drop unit 230of the node H of the ring #2 outputs the optical signal output from thering #1 to the wavelength multiplexer 240 of the node #H of the ring #2.

The dispersion compensation design is individually performed for thering #1 and the ring #2 in the communication network 800. The proceduresof the dispersion compensation design of each of the ring #1 and thering #2 are identical to those depicted in FIG. 3 and will not bedescribed.

A deviation amount d(i,j) between the residual dispersion amount andRDtgt of the optical signal inserted from the node #1 of the ring #1 andbranched from the node #j of the ring #2 may be represented by equation(3).

d(i,j)=d1(i,H)+d2(H,j)  (3)

A first term of the right-hand side of equation (3) indicates thedeviation amount between the residual dispersion amount and RDtgt of theoptical signal inserted from the node #1 of the ring #1 and branchedfrom the node #H to the ring #2. A second term indicates the deviationamount between the residual dispersion amount and RDtgt of the opticalsignal inserted from the node #H of the ring #2 and branched from thenode #j of the ring #2. The following equations (4) and (5) mayrepresent d1(i,H) and d2(H,j), respectively, of the right-hand side ofthe equation (3).

d1(i,H)=−d1(1,i)+d1(1,H)+d1(1,k+1)  (4)

where, i=1, 2, . . . , H, . . . , k

d2(H,j)=−d2(1,H)+d2(1,j)+d2(1,k+1)  (5)

where, i=1, 2, . . . , H, . . . , k

A first term of the right-hand side of equation (4) indicates thedeviation amount between the residual dispersion amount and RDtgt of theoptical signal inserted from the node #1 of the ring #1 and branchedfrom the node #i of the ring #1. A second term indicates the deviationamount between the residual dispersion amount and RDtgt of the opticalsignal inserted from the node #1 of the ring #1 and branched from thenode #H of the ring #1. A third term indicates the deviation amountbetween the residual dispersion amount and RDtgt of the optical signalinserted from the node #1 of the ring #1, passing through the nodes #2to #k of the ring #1, and branched from the node #1 of the ring #1.

A first term of the right-hand side of equation (5) indicates thedeviation amount between the residual dispersion amount and RDtgt of theoptical signal inserted from the node #1 of the ring #2 and branchedfrom the node #i of the ring #2. A second term indicates the deviationamount between the residual dispersion amount and RDtgt of the opticalsignal inserted from the node #1 of the ring #2 and branched from thenode #H of the ring #2. A third term indicates the deviation amountbetween the residual dispersion amount and RDtgt of the optical signalinserted from the node #1 of the ring #2, passing through the nodes #2to #k of the ring #2, and branched from the node #1 of the ring #2.

FIG. 10 depicts dispersion compensation design in the communicationnetwork according to the second embodiment. In FIG. 10, referencenumeral 1001 denotes characteristics of a deviation amount between theresidual dispersion amount and RDtgt of the optical signal in the ring#1. Reference numeral 1002 denotes characteristics of a deviation amountbetween the residual dispersion amount and RDtgt of the optical signalin the ring #2.

A solid line 1010 indicates a design example of the dispersioncompensation for ring #1 using the node #11 as a starting node. A dottedline 1011 indicates a design example of the dispersion compensation forring #1 using the node #11 as a starting node when it is assumed thatthe dispersion compensator included in the node #11 a fixed dispersioncompensator.

A solid line 1020 indicates a design example of the dispersioncompensation for ring #2 using the node #21 as a starting node. A dottedline 1021 indicates a design example of the dispersion compensation forring #2 using the node #21 as a starting node when it is assumed thatthe dispersion compensator included in the node #21 a fixed dispersioncompensator.

A heavy line 1030 indicates the deviation amount between the residualdispersion amount and RDtgt of the optical signal passing through thepath 830. A heavy dotted line 1031 indicates the deviation amountbetween the residual dispersion amount and RDtgt of the optical signalpassing through the path 830 when it is assumed that the dispersioncompensators included in the node #12 and the node #24 are fixeddispersion compensators.

If a fixed dispersion compensator having a step amount of Δ is used,each of the first to third terms of the right-hand side of equation (4)and the first to third terms of the right-hand side of equation (5) is±Δ/2 at maximum. The third terms of equations (4) and (5) may be set tozero depending on the setting of the dispersion compensation amount ofthe variable dispersion compensators 211 included in the node #11 andthe node #21. Therefore, the following equations (6) and (7) mayrespectively represent d1(i,H) and d2(H,j), represented in equations (4)and (5).

d1(i,H)=−d1(1,i)+d1(1,H)  (6)

where, i=1, 2, . . . , H, . . . k

d2(H,j)=−d2(1,H)+d2(1,j)  (7)

where, i=1, 2, . . . , H, . . . k

Therefore, the maximum value of the deviation amount d(i,j) between theresidual dispersion amount and RDtgt of the optical signal inserted fromthe node #i of the ring #1 and branched from the node #j of the ring #2,is constrained to ±2Δ. Although the maximum value is ±2Δ since thenumber of the rings making up the communication network 800 is two, ifthe number of the rings making up the communication network 800 isthree, four, etc., the maximum value of d(i,j) is ±3Δ, ±4Δ, etc.

The second term of equation (6) and the second term of equation (7) maybe set to zero depending on the setting of the dispersion compensationamount of the variable dispersion compensators 211 included in the node#H. Therefore, the deviation amount d(i,j) between the residualdispersion amount and RDtgt of the optical signal inserted from the node#i of the ring #1 and branched from the node #j of the ring #2 depictedin equation (3) may be represented by the following equation (8).

$\begin{matrix}\begin{matrix}{{d\left( {i,j} \right)} = {{d\; 1\left( {i,H} \right)} + {d\; 2\left( {H,j} \right)}}} \\{= {{{- d}\; 1\left( {1,i} \right)} + {d\; 2\left( {1\; j} \right)}}}\end{matrix} & (8)\end{matrix}$

Therefore, the maximum value of the deviation amount d(i,j) between theresidual dispersion amount and RDtgt of the optical signal inserted fromthe node #i of the ring #1 and branched from the node #j of the ring #2,is constrained to ±2Δ. The maximum value of d(i,j) in this case is ±Δregardless of the number of rings making up the communication network800.

For example, as indicated by a heavy dotted line 1031, if the dispersioncompensators included in the node #12 and the node #24 are the fixeddispersion compensators, the deviation amount between the residualdispersion amount and RDtgt of the light signal passing through the path830 is −4Δ/2 in the node #21.

On the other hand, as indicated by a heavy line 1030, if the dispersioncompensators included in the node #12 and the node #24 are the variabledispersion compensators 211, the deviation amount between the residualdispersion amount and RDtgt is −Δ/2 in the node #21 of the path 830.Therefore, as indicated by reference numeral 1032, the deviation amountbetween the dispersion amount and RDtgt is improved by 66% when the node#1 is equipped with the variable dispersion compensator 211.

FIG. 11 is a block diagram of a functional configuration of acommunication network according to an example of the second embodiment.As depicted in FIG. 11, it is assumed that the numbers of nodes of thering #1 and the ring #2 of the communication network 800 are four, thatthe nodes of the ring #1 are nodes N21 to N24, and that the nodes of thering #2 are nodes N23, and N25 to N27. The node N23 is a node common tothe ring #1 and the ring #2, and is a hub node (HUB) connecting the ring#1 and the ring #2.

It is assumed that a transmission path between the node N21 and the nodeN22 is a transmission path S21, that a transmission path between thenode N22 and the node N23 is a transmission path S22, that atransmission path between the node N23 and the node N24 is atransmission path S23, and that a transmission path between the node N24and the node N21 is a transmission path S24. It is assumed that atransmission path between the node N23 and the node N25 is atransmission path S25, that a transmission path between the node N25 andthe node N26 is a transmission path S26, that a transmission pathbetween the node N26 and the node N27 is a transmission path S27, andthat a transmission path between the node N27 and the node N23 is atransmission path S28.

In the communication network 800 according to the example of the secondembodiment, the node N23 acting as the hub node is defined as thestarting node of the ring #1 and the ring #2, respectively. The node N23includes the variable dispersion compensator 211. The nodes N21, N22,and N24 to N27 are nodes subjected to the dispersion compensation designusing the node N23 as the starting point and include the fixeddispersion compensators.

The transmission paths S21 to S28 are assumed to have SMF the wavelengthdispersion coefficient of 17 ps/nm/km. The number of steps of the fixeddispersion compensators included in the nodes N21, N22, and N24 to N27is assumed to be 200 ps/nm.

FIG. 12 depicts exemplary design values of the ring #1 depicted in FIG.6. FIG. 13 depicts exemplary design values of the ring #2 depicted inFIG. 11. Items 710 to 750 of FIGS. 12 and 13 are identical to thosedepicted in FIG. 7 and, therefore, are given the same reference numeralsused in FIG. 7 and will not be described.

The wavelength dispersion amounts 720 generated in the transmissionpaths S21 to S24 of the ring #1 are calculated. From the multiplicationof the spans 710 of the transmission paths S21 to S24 and the wavelengthdispersion coefficient of 17 ps/nm/km, the wavelength dispersion amounts720 generated in the transmission paths S21 to S24 may be calculated asfollows:

S23:561 ps/nm

S24:935 ps/nm

S21:1122 ps/nm

S22:1496 ps/nm

The wavelength dispersion amounts 720 generated in the transmissionpaths S25 to S28 of the ring #2 are then calculated. From themultiplication of the spans 710 of the transmission paths S25 to S28 andthe wavelength dispersion coefficient of 17 ps/nm/km, the wavelengthdispersion amounts 720 generated in the transmission paths S25 to S28may be calculated as follows:

S25:748 ps/nm

S26:1122 ps/nm

S27:935 ps/nm

S28:1309 ps/nm

The fixed dispersion compensators of the nodes N22 to N27 are selected.The ideal dispersion compensation amounts 730, the actual dispersioncompensation amounts 740, and the deviation amounts 750 from RDtgt ofthe nodes N22 to N27 may be calculated as follows:

N24: the ideal dispersion compensation amount 561 ps/nm; the actualcompensation amount 600 ps/nm; from RDtgt, the deviation amountd(N23,N24)−39 ps/nm

N21: the ideal dispersion compensation amount 896 ps/nm; the actualcompensation amount 800 ps/nm; from RDtgt, the deviation amountd(N23,N21)96 ps/nm

N22: the ideal dispersion compensation amount 1218 ps/nm; the actualcompensation amount 1200 ps/nm; from RDtgt, the deviation amountd(N23,N22)18 ps/nm

N25: the ideal dispersion compensation amount 748 ps/nm; the actualcompensation amount 800 ps/nm; from RDtgt, the deviation amountd(N23,N25)−52 ps/nm

N26: the ideal dispersion compensation amount 1070 ps/nm; the actualcompensation amount 1000 ps/nm; from RDtgt, the deviation amountd(N23,N26)70 ps/nm

N27: the ideal dispersion compensation amount 1005 ps/nm; the actualcompensation amount 1000 ps/nm; from RDtgt, the deviation amountd(N23,N27)5 ps/nm

The dispersion compensation amount is set for the variable dispersioncompensator 211 of the node N23 (on the ring #1 side), which is thestarting node. The ideal dispersion compensation amount 730, the actualdispersion compensation amount 740, and the deviation amount 750 fromRDtgt of the node N23 (on the ring #1 side) may be calculated asfollows:

N23 (on the ring #1 side): the ideal dispersion compensation amount 1514ps/nm; the actual compensation amount 1514 ps/nm; from RDtgt, thedeviation amount 0 ps/nm.

The dispersion compensation amount is set for the variable dispersioncompensator 211 of the node N23 (on the ring #2 side), which is thestarting node. The ideal dispersion compensation amount 730, the actualdispersion compensation amount 740, and the deviation amount 750 fromRDtgt of the node N23 (on the ring #2 side) may be calculated asfollows:

N23 (on the ring #2 side): the ideal dispersion compensation amount 1314ps/nm; the actual compensation amount 1314 ps/nm; from RDtgt, thedeviation amount 0 ps/nm.

From the above design, the deviation amount d(Ni,Nj) between theresidual dispersion amount and RDtgt of the optical signal inserted fromthe node Ni and branched from the node Nj consistently falls within ±200ps/nm. The calculation assumptions of d(Ni,Nj) are identical to thosedescribed in the first embodiment and will not be described.

According to the communication network of the second embodiment, sincethe hub node connecting the communication networks subjected to theindividually performed dispersion compensation design includes thevariable dispersion compensator, the maximum deviation amount betweenthe residual dispersion amount and RDtgt of the optical signaltransmitted over the communication networks may be constrained to thestep amount Δ of the fixed dispersion compensator. Therefore, thecommunication characteristics are improved by constraining thedeterioration of optical signals due to wavelength dispersion.

Since the hub node connecting the communication networks subjected tothe individually performed dispersion compensation design includes thevariable dispersion compensator and this hub node is defined as thestarting node of the dispersion compensation design of the communicationnetworks, the effect of the first embodiment is achieved and the maximumdeviation amount between the residual dispersion amount and RDtgt of theoptical signal transmitted over the communication networks isconstrained to the step amount Δ of the fixed dispersion compensator.

As described above, according to the communication network and thedesign method of the present embodiment, since the starting node of thedispersion compensation design of the communication network includes thevariable dispersion compensator, communication characteristics areimproved in the transmissions among all the nodes of the communicationnetwork. Since the hub node connecting the communication networkssubjected to the individually performed dispersion compensation designincludes the variable dispersion compensator, the communicationcharacteristics are improved in the transmissions over the communicationnetworks.

Although the ROADM communication network connecting the nodes #1 to #kin a ring shape has been described in the first embodiment, the presentinvention is generally applicable to communication networks configuredby serially connecting a starting node and multiple nodes subjected tothe dispersion compensation design using the staring node as a startingpoint.

Although a mesh communication network configured by connecting the tworing communication networks has been described in the second embodiment,a mesh communication network may generally be considered as plural ringnetworks connected to each other. Therefore, the present invention isapplicable to mesh communication networks other than the meshcommunication network described above.

All examples and conditional language recited herein are intended forpedagogical purposes to aid the reader in understanding the inventionand the concepts contributed by the inventor to furthering the art, andare to be construed as being without limitation to such specificallyrecited examples and conditions, nor does the organization of suchexamples in the specification relate to a showing of the superiority andinferiority of the invention. Although the embodiment of the presentinvention has been described in detail, it should be understood that thevarious changes, substitutions, and alterations could be made heretowithout departing from the spirit and, scope of the invention.

1. A communication network comprising: a starting node that comprises avariable dispersion compensator that performs dispersion compensation ata variable dispersion compensation amount such that a residualdispersion amount of an optical signal transmitted therethrough becomesa predetermined reference residual dispersion amount; and a plurality ofnodes that are subjected to dispersion compensation design using thestarting node as a starting point and that include fixed dispersioncompensators selected based on the reference residual dispersion amount.2. The communication network according to claim 1, wherein the fixeddispersion compensators transmit the optical signal output from thestarting node and perform dispersion compensation at a fixed dispersioncompensation amount that makes the residual dispersion amount of theoptical signal fall within a predetermined range that is based on thereference residual dispersion amount.
 3. The communication networkaccording to claim 1, wherein the variable dispersion compensatorperforms dispersion compensation at a dispersion compensation amountthat is set such that the residual dispersion amount of the opticalsignal output from a node that is among the nodes and on an input sideof the starting node, becomes the reference residual dispersion amount.4. The communication network according to claim 1, wherein the startingnode and the nodes are connected in a ring shape.
 5. The communicationnetwork according to claim 1, wherein the starting node is a hub nodethat connects to a second communication network.
 6. The communicationnetwork according to claim 5, wherein the second communication networkis a communication network comprising a plurality of nodes subjected todispersion compensation design using the hub node as the starting point.7. A communication network comprising: a hub node that connects to anynode among the starting node and the nodes constituting thecommunication network according to claim 1, and comprises a variabledispersion compensator that performs dispersion compensation at avariable dispersion compensation amount on the optical signaltransmitted therethrough.
 8. A dispersion compensation design method ofa communication network, the dispersion compensation design methodcomprising: selecting, based on a reference residual dispersion amount,respective fixed dispersion compensators included in a plurality ofnodes of the communication network, the reference residual dispersionamount being a residual dispersion amount of an optical signaltransmitted through a starting node of the communication network; andsetting a dispersion compensation amount of a variable dispersioncompensator constituting the starting node.
 9. The dispersioncompensation design method according to claim 8, wherein the settingincludes setting the dispersion compensation amount such that theresidual dispersion amount of the optical signal output from a node thatis among the nodes and on an input side of the starting node, becomesthe reference residual dispersion amount.